Enhancing NHS pension

Q: I am a 51-year-old GP locum and only joined the NHS Pension Scheme (NHSPS) in 2001. I am paying for added years at a rate of 9% until I reach pension age of 60.
Are there any other ways to maximise my pension other than changing my retirement age to 65? I read about the additional pension top-up facility but it seems very expensive.